Metering method of automatic exposure

ABSTRACT

This invention provides a metering method of automatic exposure for determining an exposure value of a picture having a subject, comprising a subject detecting step comprising using focus values of the picture to detect the subject, and an exposure value calculating step comprising using an auto exposure metering function to calculate an exposure value of the picture. The present invention can provide a simpler and smaller metering method.

FIELD OF THE INVENTION

The present invention relates to a metering method of automatic exposure used for digital still camera.

BACKGROUND OF THE INVENTION

Automatic exposure (AE), automatic white balance (AWB), and automatic focus (AF), called 3A, are the three important techniques in digital still camera (DSC) system. AE controls the amount of light reaching sensor and prevents overexposure and underexposure.

The objective of AE is to achieve a good balance of exposure in image. AE contains metering algorithm and exposure control.

First, metering algorithm estimates the amount of incident light on sensor and calculates appropriate exposure value (EV). Later in hardware control, exposure control adjusts three related devices, aperture diameter, shutter speed, and sensor sensitivity based on exposure value.

When the incident light increases, the image is over-exposed and exposure control should decrease aperture diameter, increase shutter speed, or decrease sensor sensitivity. Oppositely, when the incident light decreases, the image is under-exposed and exposure control increases aperture diameter, decreases shutter speed, or increase the sensor sensitivity. Automatic exposure control means estimating the amount of incident light and automatically adjusting exposure control.

Automatic Multi-Pattern (AMP) is one of the AE metering methods of Nikon. AMP uses a lookup table and chooses a better AE metering algorithm based on several conditions.

The general AE metering algorithms have their advantages in different scenes, so we can not adapt a metering algorithm for all scenes. For example, center-weighted metering always is used when subject locate in center, spot metering is used in backlit scenes, and average metering is used in landscape scenes where object is far away.

AMP solves this problem by deciding a suitable AE metering algorithm for each different scene. In AMP, they construct a lookup table based on a visual assessment of tens of thousands of pictures, computer analysis of the relationship between brightness patterns, optimum contrast, human evaluation, and so on. Therefore, the performance in most kinds of scenes would achieve a good result on average.

Another AE metering algorithm is disclosed by C. C. Yu in “Automatic Exposure with Fuzzy Control,” Master Thesis, Department of Computer Science and Information Engineering, National Taiwan University, 2004.

C. C. Yu describes two problems for AMP:

(1). The smooth transition is needed between each scene classification.

(2). Subject is not always in the central region.

In AMP method, subject is assumed to be in the center region. In photography, subject may not always locate in the exact center, but subject will locate in the center near left, right, top, or bottom. Moreover, if the center region also includes other insignificant information, the subject information will be diluted. Subject information is very important in scene classification of AMP. If the subject is not located in the center, we will take the wrong pictures naturally. C. C. Yu's “Subject Growing” solves this problem.

In C. C. Yu's solution, it assumes the subject always crosses the center region, and it sets the gray region as initial subject, shown in FIG. 1.

After initialization, C. C. Yu extends subject with the new regions, adjoining gray region, by considering the following conditions of new region and subject region:

(1) The difference of luminance is less than 2.0 LV.

(2) The difference of hue is less than 30 degrees.

(3) The saturation of new region is more than 0.5.

If new region conforms to three conditions, program sets it as subject region. In the end of subject growing, if the number of detected subject regions is less than 6 or more than 20, the subject is defined as default setting as the gray regions in FIG. 2.

After subject growing process, more precise subject regions are obtained and are used to calculate the contrast between subject and background for use of multi-reference table later.

In real life, the light source is unstable and changing all the time. When the measured light intensity is just near the threshold between two weather or contrast conditions, AMP metering system will changing the metering algorithms with the changing of light. If the changing is fast, we will see the display screen is flickering, and we feel jerky and uncomfortable.

Fuzzy control is the mechanism that simulates the undefined regions between defined regions. It helps to smooth the transition between each condition. C. C. Yu uses fuzzy control to smoothly transition between different weather, contrast, and subject conditions.

Obviously, subject growing process increases the probability to guess the correct subject location, and uses the subject information for more precise metering. However, the result of subject growing is not always correct. If subject growing process finds a wrong subject region, the metering algorithm will use the wrong information to take wrong pictures.

Another issue from automatic exposure is Bracket Exposure. Bracketing is a technique used to take a series of images of the same scene at a variety of different exposures that “bracket” the metered exposure. In general AE bracketing method, camera will automatically take 3 or 5 frames with exposure settings between 0.3 and 2.0 EV differences. It is useful when users are not sure exactly how the image will turn out or are worried that the scene has a wide dynamic range.

When we take pictures using AE bracketing, we next select our favorite one from bracketing pictures. An “Optimal Exposure Selector (OES)” objectively selects one favorite picture from AE bracketing pictures, which differ in exposure only. About the benefits, OES saves the effort that users need to select, and saves the memory space that bracketing pictures occupy more spaces.

To select the favorite picture, researchers analyze the factors of person perceptions, such as brightness, contrast, and colorfulness.

In decision making, the prior art OES analyzes the factors that are related to the person perception and are influenced by exposure control. The following three factors are considered:

1. Intensity Mean

Intuitively, the light intensity factor is the most related to exposure control. In implementation, we take the mean of light intensity to judge the quality. The larger of the mean, the bright the picture is.

2. Standard Deviation and Entropy

Besides the light intensity factor, C. C. Yu also considers the contrast factor. Different exposure controls will result in the different contrasts, and people prefer the high-contrast picture. To analyze this factor, he calculates the standard deviation and entropy of histogram. The larger standard deviation, the wider distributed of histogram is, thus higher contrast. The larger entropy, the more uniform of histogram is.

3. Colorfulness

People like colorful image. In application, C. C. Yu thinks colorfulness as the distance from the pixel to origin point in CbCr coordinates. The smaller the distance, the grayer the pixel is.

However, the matching ratio (i.e., selecting the right picture) of the prior art OES is not good enough.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a metering method of automatic exposure which is simpler and smaller than the disclosed prior art.

In accordance with an aspect of the present invention, there is provided a metering method of automatic exposure for determining an exposure value of a picture having a subject, comprising a subject detecting step comprising using focus values of the picture to detect the subject, and an exposure value calculating step comprising using an auto exposure metering function to calculate an exposure value of the picture.

The subject detecting step comprising finding out ten segments which have maximum focus values; calculating numbers (l, c, r) respectively represented numbers of the segments which contain maximum focus values in a left region, a center region and a center region of the picture respectively; and determining a location of the subject. The subject is in the center region, if c>4, or ((c>l) and (c>r)). The subject is in the left region, if the subject is not in the center region and (l>c) and (l>r). The subject is in the right region, if the subject is not in the center region and (r>c) and (r>l).

The exposure value calculating step comprising calculating an average luminance of the regions; sorting the luminance of the regions and calculating a Contrast-Ratio; finding interesting regions; and calculating the exposure value according to the equation: Exposure value=(1−Contrast_Ratio)×subject+Constrast_Ratio×interesting regions.

The Constrast_Ratio is obtained according to a Contrast Ratio function; wherein the Contrast Ratio function determines the Constrast_Ratio by a contrast value which is obtained by subtracting a minimum luminance among the regions from a maximum luminance among the regions. If the luminance of the subject region is close to the maximum luminance, the interesting region is a bright region. If the luminance of the subject region is close to the minimum luminance, the interesting region is a dark region.

This invention also provides an optimal exposure selecting method of bracket exposure, for selecting an optimal exposure picture having an optimal exposure value among a plurality of pictures having the same scene as the picture but having different exposure values, wherein the method selects the optimal exposure picture according to at least two factors: image details and ratio of non-saturating pixels.

The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a gray center region of C. C. Yu;

FIG. 2 schematically illustrates gray regions of C. C. Yu's default subject;

FIG. 3 is the flowchart of the metering method of the invention;

FIG. 4 shows the flowchart of subject detection step of this invention;

FIG. 5(a) shows a center region of DSC sensor of this invention;

FIG. 5(b) shows a right region of DSC sensor of this invention;

FIG. 5(c) shows a left region of DSC sensor of this invention;

FIG. 6 shows the flowchart of using an AE metering function of this invention to calculate an exposure value;

FIG. 7 is a transformation between Contrast and Contrast_Ratio of this invention;

FIG. 8(a) shows a exposure result using prior art method;

FIG. 8(b) shows a exposure result using the method of this invention;

FIG. 9(a) shows another exposure result using prior art method;

FIG. 9(b) shows another exposure result using the method of this invention;

FIGS. 10(a)-10(c) show bracketing images.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.

The metering method of this invention contains two parts:

(1) Subject detection, and

(2) Exposure value calculation based on a AE metering function

Subject detection is different from subject growing and does not set center as subject region. We consider the focus information to decide the subject is in the center, left, or right. Moreover, we propose a new AE metering function to replace AMP table. Without looking up table and saving table, the new AE metering function is simple and small.

FIG. 3 is the flowchart of the metering method of the invention. First, light information from sensor of DSC is received and checked if the focus information is available or not. If focus information is available, we use subject detection to choose the subject region, otherwise, we use default center region. Finally, with subject region, we calculate the EV by new AE metering function.

In preview mode, AF will not work at the time to save the power, and focus information is not available. We use center as default subject and keep a reasonable exposure. In capture mode, AF works and focuses on the subject, we can detect subject more precise to take better pictures.

Without subject growing process and guessing subject always in the center, we use the focus information, if available, to detect subject location. Because the AF focuses on the subject with high probability, we optimize the exposure quality of subject region, containing high focus value. Even if the AF does not focus on the correct subject, the focused region is still clearer than others, and it is reasonable to optimize exposure quality for clear regions. FIG. 4 shows the flowchart of subject detection step of this invention. The sensor of DSC is defined as regions 1-8 and classified as a center region, a right region and a left region. FIG. 5(a) shows the center region of sensor. FIG. 5(b) shows the right region of sensor. FIG. 5(a) shows the left region of sensor.

First, we get the focus values of each segment from AF. Focus value represents the levels of high frequency in the segments, and high focus value means high frequency and clear region. We find out ten segments, which contain maximum focus values and their light intensities are less than a luminance threshold. Next, we calculate (c, 1, r), the numbers of maximum segments in each region. FIG. 3.4 shows three gray regions of possible subject locations. The rules of region decision are:

(1) Center region: (c>4) or ((c>l) and (c>r)).

(2) Left region: center condition fails and (l>c) and (l>r).

(3) Right region: center condition fails and (r>c) and (r>l).

(4) Default region: otherwise.

The rules of region decision consider three cases:

-   -   (1) The center region still has the highest probability to         contain subject. We detect left or right region only when center         region has bad focus and light intensity.     -   (2) The right or left region needs rigorous condition to be         subject. It should contain more focus regions than others and         center fails in Case one.     -   (3) If above conditions both fail, then we choose center as         subject region.

Finally, subject detection process passes subject region information to AE metering function.

FIG. 6 clearly shows the flowchart of using an AE metering function to calculate an exposure value. First, light information and subject region are obtained to calculate average luminance of each region. As in FIG. 5, for different subject regions, function calculates different Regions 1˜8 and subject region. Second, it sorts Regions 1˜8 and calculates the contrast by subtracting min region from max region. The contrast ratio is calculated by the transformation in FIG. 7. Contrast is concerned when it is larger than contrast threshold (3LV), and maximum contrast ratio is 40% to protect the importance of subject information.

If contrast is larger than contrast threshold, we find the interesting regions depending on the order of subject region in sorted regions. If subject region is close to the maximum part, we take bright region as interesting region. Otherwise, if the subject region is close to the minimum part, we take the dark region as interesting region. Finally, with the value of subject, contrast ratio, interesting region, and cut process, function calculates the exposure by the Equation: Exposure value=(1−Contrast_Ratio)×subject+Constrast_Ratio×interesting regions

FIG. 8(a) shows the exposure result using prior art method and FIG. 8(b) shows the result using the method of this invention. In FIG. 8, subject is not in the center. It is obvious that without focus information(FIG. 8(a)), the prior art method emphasizes the center region and overexposes the subject. With focus information (FIG. 8(b)), the subject is emphasized correctly and has a better exposure result.

FIG. 9(a) shows another exposure result using prior art method and FIG. 9(b) shows the result using the method of this invention. In FIG. 9, subject is in the center, but left or right region has a high contrast object which has high focus values. We see that with focus information, subject detection still does not lose the subject, even if the left object have more high frequent regions.

In AE bracketing, we use the subject detection process of this invention as described above and modify the factors of OES for more precise selection.

1. Intensity Means

Intensity means of subject and global are still important factors and directly represent the AE performance. We emphasize the intensity mean as prior art OES.

2. Intensity Standard Deviation and Entropy

Intensity standard deviation represents the contrast of pictures, and the higher contrast, the more people prefer. In fact, by the observation from histograms of pictures, different exposure little affects the standard deviation of histogram. People do not prefer the scenes with highest standard deviation, because other factors are more important. We still consider the standard deviations of subject and global, but we give them less weight in overall consideration.

3. Colorfulness

Most people like colorful pictures, but the relationship between colorfulness and exposure setting is less, so we give colorfulness less weight in overall consideration.

4. Detail

One of the important purposes in AE metering method of this invention is to show as much detail as possible. Detail factor is not considered in the prior art OES, but we can show how importance of details from FIGS. 10(a)-10(c). FIGS. 10(a)-10(c) are the bracketing images. FIG. 10(a) has higher scores in light intensity, but the details are less than FIG. 10(b). People prefer 10(b) because the details are clear. We consider the detail factor is important, therefore we give it 10 scores at most.

5. Non-Saturating Ratio

Besides the light intensity and detail considerations, another important purpose of AE is to prevent the image underexposure or overexposure. In other words, the saturating pixels should not be many, so the ratio of non-saturating pixels is another important factor in exposure selection.

We take 30 sets of AE bracketing picture from Sony F828 and Fujifilm F601. The image sets contain the scenes of outdoor, indoor, landscape, portrait, and so on. Table I show the matching ratio of the OES factors of this invention and C. C. Yu's results. TABLE I Item Matching Mismatching Ratio Intensity Mean 24 6 80% Intensity St. Dev. 10 20 33% Colorfulness 6 24 20% Detail 17 13 57% Non-Saturating Ratio 21 9 70% New OES Result 26 4 87% C.C. Yu's OES Result 19 11 63%

As shown in experiments, the modified OES can select the better exposed pictures according to details and non-saturating ratio.

While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. 

1. A metering method of automatic exposure for determining an exposure value of a picture having a subject, comprising: a subject detecting step comprising using focus values of the picture to detect the subject; and an exposure value calculating step comprising using an auto exposure metering function to calculate an exposure value of the picture.
 2. The metering method of automatic exposure according to claim 1, wherein the picture comprising a plural segments and the subject detecting step comprising: finding out ten segments which have maximum focus values; calculating numbers (l, c, r) respectively represented numbers of the segments which contain maximum focus values in a left region, a center region and a right region of the picture respectively; and determining a location of the subject.
 3. The metering method of automatic exposure according to claim 2, wherein: (1) the subject is in the center region, if c>4, or ((c>l) and (c>r)); (2). the subject is in the left region, if the subject is not in the center region and (l>c) and (l>r); (3) the subject is in the right region, if the subject is not in the center region and (r>c) and (r>l).
 4. The metering method of automatic exposure according to claim 1, wherein the exposure value calculating step comprising: calculating an average luminance of the regions; sorting the luminance of the regions and calculating a Contrast_Ratio; finding interesting regions; and calculating the exposure value according to the equation: Exposure value=(1−Contrast_Ratio)×subject+Constrast_Ratio×interesting regions.
 5. The metering method of automatic exposure according to claim 4, wherein the Constrast_Ratio is obtained according to a Contrast Ratio function; wherein the Contrast Ratio function determines the Constrast_Ratio by a contrast value which is obtained by subtracting a minimum luminance among the regions from a maximum luminance among the regions.
 6. The metering method of automatic exposure according to claim 5, wherein: if the luminance of the subject region is close to the maximum luminance, the interesting region is a bright region; if the luminance of the subject region is close to the minimum luminance, the interesting region is a dark region.
 7. An optimal exposure selecting method of bracket exposure, for selecting an optimal exposure picture having an optimal exposure value among a plural pictures having the same scene and different exposure values, wherein the method selects the optimal exposure picture according to at least two factors: image details and ratio of non-saturating pixels. 